Wright Track makes the most accurate one I've seen. It's a resin kit.
The Athearn one is okay, but the placement of the windows is off. If that doesn't bother you (and it doesn't bother me much), then Athearn is where it's at (until Walthers makes one).
Out of the box, the Athearn is easy to build. Improvements would be painting the porch red, the stairs yellow, and add some metal wheelsets and Kadee couplers. I'd prefer not to have a running board/roofwalk on mine, as well as ladders, since later cabooses I've seen didn't have them.
Orange Grove Hobby Shop, Orange Grove, MS,
Link to contact information here, had a couple of the last run of RTR Athearn Southern cabooses in stock (though I bought one of them).
Hub City Hobby, in Hattiesburg, MS, had the last run of Athearn blue box kits with a Southern caboose (X591, I think) as recently as November, when I was there last. They also have some Southern stuff.
I've taken to building some of my own (using Accurail 50' boxcars or MDC/Roundhouse undecs off of ebay), as growing up, Southern 50' boxcars were as ubiquitous as leaves on a tree. Unfortunately, we don't see much in the way of Southern as other railroads now. You can still find new the Athearn 40' modernized boxcars, the Walthers 7000cuft woodchip hopper and SIECO pulpwood flat, the 50' waffle boxes from Exactrail, 100tn coal hoppers from Bowser, and perhaps something else here and there.